With the Blu-ray Disc, which is a next-generation optical disk, just as with ordinary optical disks, a protective film composed of a polycarbonate is bonded to the information recording layer in order to protect this layer. The recorded information on a Blu-ray Disc is extremely large capacity and high density, and even a slight scratch to the protective film can result in errors in the reading or writing of information, so at present Blu-ray Discs are housed in a cartridge to prevent such scratching.
However, a cartridgeless type bare disk is desired in order to make the Blu-ray Disc a more compact medium and to reduce manufacturing costs.
To this end, it is possible to form a hard coat layer on the protective film used for the information recording layer, as disclosed in Japanese Laid-Open Patent Application 2003-157579, but with protective films featuring a conventional hard coat layer, considerable warpage is caused by the curing shrinkage of the hard coating agent, which leads to warpage of the disk and problems with signal characteristics. Therefore, to suppress this warpage, the hard coat layer had to be made thinner, and more specifically, the thickness had to be reduced to between 1 and 3 μm. With such a thin hard coat layer, however, the surface is not hard enough to prevent scratching.
It is also possible to use the resin composition disclosed in Japanese Laid-Open Patent Application 2000-273272 as a hard coating agent, but here again, the hard coat layer has to have a certain thickness in order to achieve an adequate surface hardness in the resulting hard coat layer, and it is therefore difficult to suppress warpage in the protective films and in the resulting disks.